Photovoltaic solar panels, also referred to simply as solar panels, are generally of two basic designs. One design employs crystalline silicon wafers connected together and embedded in a laminating film. The laminating film and the wafers embedded therein are typically sandwiched between two lights, or panels, of glass, a polymeric material or other suitable materials.
The second solar panel design, which is of primary interest herein, employs one of amorphous silicon, cadmium-telluride (Cd—Te) or copper-indium-diselenide, CuInSe2 (commonly referred to as “CIS”), or a similar semiconductor material such as mentioned below, which is deposited on a substrate in a thin film. These thin film photovoltaic materials are typically deposited in a thin film on a glass substrate by a method such as sputtering, PVD or CVD. The individual photocells are typically formed by a laser etching process, and are connected together by suitable circuitry, such as a bus bar. The bus bar transfers to a storage device the electrical current output from the photocells. The thin film photovoltaic material and associated circuitry may be covered by a sputtered layer of aluminum, which acts to protect the underlying structures. To complete the construction, an assembly adhesive is applied over the photovoltaic material, associated circuitry, and any protective layer which is present, and a backing material is applied. The backing material is typically glass, but may be metal, a composite or a plastic material.
In addition to the above noted CIS, other combinations of Group I, Group III and Group IV (referred to as I-III-IV) semiconductor materials have been used and/or proposed for use as photovoltaic materials. A number of different I-III-VI semiconductor materials have been proposed for use in photovoltaic cells. Some examples include AgInS2, AgGaSe2, AgGaTe2, AgInSe2, AgInTe2, CuGaS2, CuInS2, CuInTe2, CuAIS2, and CuGaSe2. Most attention, however, has been focused on CIS and variations of CIS in which a portion of the indium is replaced with one or more of aluminum and gallium and/or a portion of the selenium is replaced with sulfur and/or tellurium. Two promising variations of CIS that have been proposed include CulnxGa1-xSe2 (commonly referred to as “CIGS”) and CuInxGa1-xSeyS2-y (commonly referred to as “CIGSS”). These and other I-III-VI semiconductors may be used in photovoltaic cells, as is known in the art.
The circuitry, such as a bus bar, which collects the electrical current generated by the solar panel must be connected by wiring to a suitable storage device, such as a battery. Such wiring may be referred to as a “module wire” or “module lead”. The module wire must exit the solar panel at some point. Additional adhesive or sealant material is needed to seal around the module wire exiting the solar panel. The adhesive used for sealing around module wires may be the same as, or may differ from, the assembly adhesive used to attach the backing material to the solar panel.
Solar panels are used outdoors, and so are exposed to the elements, including wind, water and sunlight. Solar panels are deleteriously affected primarily by moisture which may permeate into the panel, reaching the electrical connections or the photovoltaic materials. Water penetration into solar panels has been a long-standing problem. Thus, various attempts have been made to reduce the moisture vapor transmission rate (MVTR). Solar panels may also be deleteriously affected by wind and sunlight, which may result in failure of the adhesive layer. Wind causes obvious physical damage, and sunlight results in heating of the solar panel and exposure to ultraviolet (UV) radiation. Operating temperatures of solar panels have been measured as high as 110° C. Thermoplastic adhesives soften at elevated temperatures and are susceptible to UV-induced breakdown. Many thermosetting materials suffer from unacceptably high MVTR.
One presently used assembly adhesive is ethylene vinyl acetate (EVA). The EVA is applied to the solar panel together with a peroxide which can crosslink the EVA. The EVA is then cured in place on the solar panel by application of heat or radiation, which causes the peroxide to crosslink the EVA. Crosslinked EVA provides high strength, but suffers from a relatively high MVTR.
Module wire sealing materials suffer from the same problems as do the assembly adhesives. Presently used module wire adhesive/sealants include epoxy compounds and hot melt butyl compounds. Epoxy compounds suffer from relatively high MVTR. The hot melt butyl systems suffer from the inability to achieve high strength since they are generally supplied as a thermoplastic material and they lose strength as temperatures increase, as noted above.
The problems of excluding moisture from solar panels, and of finding adhesives with suitably low MVTR properties, in addition to the other properties required of such adhesives, have been long standing. Many attempts have been made to provide suitable adhesive materials. However, none has satisfactorily provided both the required strength and related properties, and the required low MVTR properties. The present invention provides a solution to this problem by providing a low MVTR adhesive material suitable for use in a solar panel.